The easiest way is of course to have it done professionally . But, some of us like to do it ourselves, so, here is the best way to adjust your headlights yourself.
Step one is for 2 headlight systems:
park on a level surface, this is highly critical, as most constituencies have certain requirements, and the following will generally fit all requirements, but, check your local requirements, please.
so, for 2 headlights, you need your car on a flat surface, and you preferably need any vertical flat surface in front of the car. now, park the car close as possible to the vertical surface and turn on the headlights. mark with a marker or masking tape or whatever where the headlights hit the vertical surface. you want to mark the center (approx) of the beam. This will work for left or right hand drive by the way...
now, back the car up approx 3 feet, this is where the low beams should be aimed. Now, back up to around 30 feet, this is where the high beams (approx) should be aimed. In a 2 headlight system, you will need to approximate the beams to the best option between these two marks. As you will see, in a 2 headlight system, there needs to probably be a bit of a compromise in this method, that a professional adjustment can make more accurate, but, as a do it at home method, this should be close enough to pass many inspections.
and, now easier, the 4 headlight system. You need to do the same as above, but, aim each beam type as above, low beams to approx 3 feet, and high beams to about 30 feet. Following this will not guarantee passing any test, but, has a higher chance of not blinding oncoming traffic.
The above takes into account varying ride hights and can compensate for any modifications.
As a side note, before you do this, it is a good idea to make sure the tire pressures are accurate, and you surfaces are flat (horizontally) and also that you vertical surface is vertical, fences or garage doors work well for this.
Again, to pass a government safety test, the best resolution is to have the adjustments done professionally.